Multi-terminal HVDC (MTHVDC) interaction studies using small signal stability assessment

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Ma, Shaohua
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With the development of technology, the HVDC system becomes more and more attractive in modern power systems. Tapping on existing HVDC lines becomes a practical option to deliver reliable power to the customer. However, there are potential interactions in the Multi-terminal HVDC (MTHVDC) systems. To investigate the interactions in MTHVDC systems, a detailed linearized Line Commutated Converter (LCC) based three-terminal HVDC system model is developed. The simulation results are benchmarked with the corresponding PSCAD/EMTDC model. This thesis proposes a comprehensive study procedure to perform the MTHVDC system interaction study. Using the proposed study procedure, the complex oscillation modes of the two proposed three-terminal HVDC test systems have been evaluated. The results show that machines at different terminals interact with each other through the three-terminal HVDC systems. The results demonstrate that the damping ratio for electromechanical oscillations can be improved by changing the HVDC controller parameter. In addition, the results also show that different controller arrangements have an impact on each oscillation mode. This thesis also investigates the impact of the system strength on MTHVDC controller interactions. Two MTHVDC control schemes with various system configurations have been studied with various AC system strength conditions. The results show that AC system strength has a significant impact on its own terminal controller state variable dominated oscillation mode. Particularly for the DC voltage controller state variable dominated oscillation mode. The results also show that higher system strength does not always positively improve the oscillation damping ratio. Other parameters, such as system configuration, controller parameter as well as the system operating point also impact the controller interaction oscillation. In general, the developed model, analytical techniques, and the study procedure proposed in this thesis can be used to analyze power system high-frequency interactions of HVDC systems, controller interactions of the MTHVDC system, and FACTS devices.
Multi-Terminal, HVDC, Small Signal
IET AC/DC,2019, UK